The present invention relates to overcompensated feedforward amplification method and apparatus and, more particularly, to method and apparatus for providing substantially distortion-free amplification of input signals in overcompensated feedforward amplifier arrangements using overdistorted main amplifiers.
In wideband coaxial cable transmission systems in which the signal must pass through a large number of repeater stages between transmitting and receiving stations, the distortion generated by the power amplifying stages in each repeater plays a significant role in the degradation of signal quality. It is desired, therefore, to keep the distortion generated at each repeater stage as low as possible.
Prior art feedforward amplifier techniques, such as disclosed by H. Seidel in U.S. Pat. No. 3,471,798 issued Oct. 7, 1969, reduce the distortion generated by the power amplifying stage. In the Seidel patent, an input signal is divided into two portions, one portion of the signal being coupled to a main power amplifier that introduces distortion due to its nonlinearity with the remaining portion being coupled to a parallel second signal path. The amplified signal in the first signal path, comprising a linear and distortion component, is attenuated and combined with the undistorted signal in the second signal path such that the linear signal components cancel. The resultant signal, comprising solely a distortion component, is reamplified by an auxiliary power amplifier to produce a distortion signal equal in magnitude to the distortion component at the output of the main power amplifier. When this distortion signal at the output of the auxiliary amplifier is combined with the linear and distortion components at the output of the main power amplifier, the distortion components are cancelled. The resultant feed-forward amplifier output signal is thus representative of the distortion-free linear component at the output of the main amplifier. Since, however, the auxiliary amplifier amplifies only a distortion component, the magnitude of the feedforward output signal is determined by the amplification factor of the main power amplifier and the signal loss in the output coupler. Thus, since the auxiliary amplifier does not contribute to the feedforward amplifier power output, the output power is thus limited by the power output capabilities of the main power amplifier, and is less than the output power of the main amplifier, due to the signal loss in the output coupler.
In U.S. Pat. No. 3,725,806 issued Apr. 3, 1973 to S. Darlington, a distortion reducing amplifier is disclosed in which an auxiliary network generates a compensating signal comprising both linear and distortion components. When, however, the linear and distortion components in the auxiliary network are combined with the linear and distortion components at the output of the main distortion producing power amplifying network to cancel the distortion components, the linear components are of such phase relationship that the resultant linear component is less than the linear component at the output of the main power amplifying network. Therefore, the auxiliary network reduces the power output of the main power amplifying network.
In U.S. Pat. No. 3,873,936 issued on Mar. 25, 1975 to Y-S Cho, the distortion generated due to the nonlinear characteristics of the power amplifier in a repeater network is substantially reduced by generating a compensating signal in a feedforward amplifier network. In the feedforward network, an auxiliary power amplifier having substantially the same gain and distortion characteristics as the main power amplifier generates a compensating signal having a linear component and a distortion component equal in magnitude to the linear component and the distortion component, respectively, of the output signal of the main power amplifier. The phase relationship between the linear and distortion components at the output of the auxiliary amplifier is, however, opposite to the phae relationship between the linear and distortion components of the power amplifier. Thus, when the compensating signal is combined with the output of the main power amplifier, the distortion components are substantially cancelled and the linear components are added. The substantially distortion-free output signal, therefore, has greater power than was obtainable in prior art feedforward distortion reduction networks.
The ability to relax the requirements within components of the prior art overcompensated conventional or parallel feedforward amplifier arrangements, as shown generally in U.S. Pat. Nos. 3,471,798 and 3,873,936, respectively, for an advantageous reduction in repeater cost and size, while accomplishing the objectives of such arrangements, still remains a problem.